This combination of undated photos shows Tamerlan Tsarnaev, 26, left, and Dzhokhar Tsarnaev, 19. Boston officials said Sunday the two suspects in the Boston marathon bombings appeared to be planning more attacks but likely acted alone, while Republican lawmakers questioned why the FBI hadn't taken a harder look at one of the ethnic Chechen brothers after Russian officials sounded alarm bells about him.(AP Photo/The Lowell Sun & Robin Young)

WASHINGTON - The surviving Chechen brother suspected in the bloody Boston marathon bombings is reportedly awake and communicating with investigators in writing amid speculation that he and his deceased sibling had plotted further terrorist attacks.

Both ABC and NBC News reported that Dzhokhar Tsarnaev, who suffered a gunshot wound to his neck thought to be the result of a suicide attempt, was conscious and responding to investigators sporadically in writing.

The networks cited law enforcement officials as saying the U.S. citizen was being asked about possible accomplices and undetonated bombs.

Earlier in the day, officials said the 19-year-old Tsarnaev was too seriously wounded to be questioned. They also said there was no indication yet that Tsarnaev and his now-deceased older brother, Tamerlan, were part of a broader terrorist conspiracy.

"All of the information I have is they acted alone," Boston Mayor Thomas Menino told ABC's "This Week." The police chief in suburban Watertown said the same thing a day earlier.

Boston's police commissioner, meantime, said the crime scene following a wild firefight in Watertown early Friday suggested the suspects were plotting further attacks.

Edward Davis told CBS's "Face the Nation" that the scene of the gun battle was littered with unexploded improvised explosive devices.

"We have reason to believe, based upon the evidence that was found at that scene — the explosions, the explosive ordinance that was unexploded and the firepower that they had — that they were going to attack other individuals," he said.

In the U.S. capital, meantime, Republican legislators said they suspected the brothers, who immigrated to the United States in 2002, could have been part of a larger terrorist conspiracy. Like all Chechens, the brothers were Muslim.

They also criticized the FBI for not taking a closer look at Tamerlan Tsarnaev, 26, a permanent U.S. resident, after Russian officials raised alarm bells about him two years ago.

"This is the latest in a series of cases like this ... where the FBI is given information about someone as being a potential terrorist," Pete King, the Republican head of the House of Representatives' subcommittee on counterterrorism and intelligence, told "Fox News Sunday."

"They look at them, and then they don’t take action, and then they go out and commit murders."

The FBI has acknowledged it questioned Tamerlan Tsarnaev in 2011 at the request of the Russian government, but concluded he was not a threat and had no links to terrorist organizations.

Since then, however, Tamerlan Tsarnaev travelled back to the Russian regions of Dagestan and Chechnya during a six-month trip last year. Friends and family members have said publicly that the elder Tsarnaev brother returned from that trip noticeably different, with a much more religious mindset.

Federal authorities are now looking into that foray to determine whether Tsarnaev had any contact with Chechen extremists in the region. The major active group there — the Caucasus Emirate, a small band of fierce Islamic insurgents — has denied any involvement in the bombings.

“The Caucasian Mujahadeen are not carrying out attacks against the United States of America," the group said in a statement.

"We are at war with Russia, which is responsible not only for the occupation of the Caucasus but unbelievable crimes against Muslims."

Nonetheless, another Republican legislator said Sunday he suspected the elder Tsarnaev was radicalized during that trip to his homeland and was "very probably" trained by Islamic extremists.

"I personally believe that this man received training when he was over there, and he radicalized from 2010 to the present," Michael McCaul, chairman of the House homeland security committee, said on CNN's "State of the Union."

"One of the first things he does (upon his return) is put up a YouTube website throwing out a lot of jihadist rhetoric. Clearly something happened, in my judgment, in that six-month timeframe — he radicalized at some point in time. Where was that and how did that happen?"

The Texas lawmaker also questioned why the FBI didn't scrutinize Tsarnaev more closely upon his return to the United States last year.

"If he was on the radar and they let him go, if he was on the Russians’ radar, why wasn't a flag put on him, some sort of customs flag?" McCaul asked.

Another Republican legislator, Mike Rogers — chairman of the House intelligence committee and a former FBI agent — defended the FBI, and said he suspected Tamerlan Tsarnaev may have made the trip to Russia last year under an alias, slipping under the radar of authorities.

Tamerlan Tsarnaev died early Friday after waging the gunfight with police alongside his younger brother. Dzhokhar Tsarnaev eluded police for hours before being captured hiding in a pleasure boat in a nearby residential backyard.

The New York Times reported Sunday that officials believe Dzhokhar Tsarnaev tried to kill himself.

His neck injury "had the appearance of a close range, self-inflicted style" wound, a law enforcement official told the Times. "He's not in good shape."

Three people were killed, including an eight-year-old boy, and more than 180 were injured when two crudely designed bombs were detonated about 10 seconds and 90 metres apart at the finish line of the famed marathon last Monday.

The bombings kicked off a frightening week for Americans still traumatized by the horrors of the Sept. 11, 2001 terrorist attacks. The state of high anxiety didn't ease until police took a bloodied Dzhokhar Tsarnaev into custody just after nightfall on Friday night, prompting the city of Boston to erupt in celebration.

The hunt for the culprits intensified after the FBI released video surveillance images Thursday that showed the Tsarnaev brothers milling about the finish line of the famed marathon carrying backpacks.

Jeff Bauman, who had both legs blown off in one of the blasts, provided a detailed description to investigators of one of the suspects that helped them zero in on the brothers from a sea of faces in the crowd.

Within hours of releasing their images, investigators were flooded with tips from the public about their identities, setting off a stunning chain of events that began when a security officer at the Massachusetts Institute of Technology was gunned down as he sat in his cruiser.

The gunfight broke out a couple of hours later following a police chase in which the suspects hurled explosives out the windows of a carjacked Mercedes SUV. That firefight ended when Dzhokhar Tsarnaev allegedly drove over his brother, whom police had pinned down on the ground before leaping out of the way, and dragged his body underneath the vehicle for a short distance as he escaped the scene.

By daybreak, Tamerlan Tsarnaev was dead, his kid brother was on the loose and the city of Boston and its suburbs were on lockdown as police engaged in one of the biggest manhunts in American history.

Troubling portraits continue to emerge of the social isolation and increasing radicalism of Tamerlan Tsarnaev, in particular, amid repeated suggestions from friends and family that he may have led his younger brother astray.

The elder Tsarnaev was married to an American woman who converted to Islam for him. They had a three-year-old child but Katherine Russell's parents released a statement saying they "never really knew" their son-in-law.

While Dzhokhar Tsarnaev had a wide circle of friends and has almost universally been described as kind-hearted and intelligent, his brother apparently struggled to fit in and grew increasingly embittered about the United States.

The surviving Tsarnaev brother was to be interrogated without authorities advising him of his right to remain silent and to obtain a lawyer, a warning typically given to crime suspects in the United States.

Called Miranda rights, authorities can revoke them if they deem the suspect poses a continuing threat to public safety. In Tsarnaev's case, that likely means investigators want to ensure there are no unexploded devices or accomplices still at large.

The decision has nonetheless raised the hackles of the American Civil Liberties Union, which has branded the move "un-American" and warned it would make it more difficult to "obtain fair convictions."

Charges hadn't yet been laid against Dzhokhar Tsarnaev. The most serious federal charge the teen could face would be the use of a weapon for mass destruction to kill people, which carries a possible death sentence.